locked Re: cover photo
I found that as long as I kept the browser window approximatelyI don't think that's a glitch: I think it is intended that the height available to the photo not grow taller as the window grows wider. I think it is a reasonable compromise between traditional 4:3 or 5:4 (nearly square) monitors and today's 16:9 or even wider widescreen monitors.
If the photo height grew proportionate with the width that would take up way too much room in a window shaped to fit a widescreen monitor. Conversely if the width of the photo did not expand with the width of the window, then you'd end up with a "postage stamp" image within a wide bar of background color.
Given this, as guidance to moderators I'd say pick your image to suit the fact that this vertical cropping will occur: Keep the interesting part of the photo to the center. In other words, rendering text into the photo or otherwise trying to dense-pack information there is not going to work.
I realize that was a favored technique in Yahoo Groups. Both to make a larger presentation and to overcome the 2000 character limit for the description text. One of the larger examples I've seen is this one:
That's multiple photos and text all composed together into a single very large homepage image.
As guidance to Groups.io I'm not entirely sure what to suggest. Large displays like the above are almost certainly better handled via HTML support within the description, hopefully without a too-small limit on the number of characters.
Possibly add an option to disable the width matching so that something like logo art can be used as the group's image, knowing that it will be flanked with background color but without the danger of it being cropped.